1. Field of the Invention
The present invention relates to a bus loop power interface, and more particularly, to a bus loop power interface and method for an instrumentation bus.
2. Statement of the Problem
Flowmeters are used to measure the mass flow rate, density, and other characteristics of flowing materials. The flowing materials can comprise liquids, gases, combined liquids and gases, solids suspended in liquids, and liquids including gases and suspended solids. For example, flowmeters are widely used in the well production and refining of petroleum and petroleum products. A flowmeter can be used to determine well production by measuring a flow rate (i.e., by measuring a mass flow through the flowmeter), and can even be used to determine the relative proportions of the gas and liquid components of a flow.
In many process control or industrial automation settings, a bus loop (or instrumentation bus) is used to connect to various devices, such as sensor devices and other instrument devices. The bus loop is commonly used to deliver electrical power to the various attached instrument or sensor devices. In addition, the bus loop is also commonly used to communicate data both to and from the sensor or instrument device. Therefore, the bus loop is connected to a master device that can provide regulated electrical power over the bus and that can perform communications over the bus. The master device can send commands and/or programming, data, calibrations and other settings, etc., to the sensor and instrument devices. The master device can also receive data from the sensor and instrument devices, including identification data, calibration data, measurement data, operational data, etc.
FIG. 1 shows a two-wire bus loop 1 according to the prior art. The bus loop 1 can include a master device 2 that operates the bus loop 1, multiple sensor or instrument devices 3-5, and terminators 6. The master device 2 can pass digital communication signals to external devices, such as to monitoring and control stations, for example. The master device 2 is connected to an electrical power source and provides electrical power over the two-wire bus loop 1. The master device 2 typically provides electrical power that is current limited, voltage limited, and power limited.
The bus loop 1 can comprise a FIELDBUS bus loop, for example. The term FIELDBUS refers to a two-wire instrumentation bus standard that is typically used to connect together multiple instruments and is further capable of being used to provide digital communications between instruments. Alternatively, the bus loop 1 can comprise other buses, such as PROFIBUS, HART, a 4-20 mA bus loop, etc.
FIG. 2 shows a prior art power regulation scheme for a prior art bus device 3 connected to the bus loop 1. The prior art bus device 3 includes voltage regulator 8 and a load 9 in series with the voltage regulator 8. The voltage regulator 8 maintains a constant voltage level to the load 9.
However, this prior art power regulation has drawbacks. The maximum electrical power (P) provided in this prior art arrangement comprises the output voltage of the series regulator (V) multiplied by the loop current (I). A loop interface that uses this kind of linear regulation has poor power transfer because the voltage parameter V in the available power equation (P=V×I) is essentially fixed. In addition, the bus voltage dips when the loop current increases, due to the relatively high loop impedance.
One possible solution could be the use of a switch mode power supply (SMPS). A SMPS is also referred to as a switched mode power supply or a switching power supply. A SMPS generates a time varying signal from the DC bus voltage, performs a voltage step-up conversion, and converts the resulting time-varying signal back to a DC voltage that can have a higher voltage level than the original DC bus voltage. As a result, the V term in the power equation can be increased.
However, the drawback of a SMPS device is that a SMPS device typically has a low input impedance characteristic. This is not compatible with a device powered from the bus loop 1, where the loop impedance is kept high in order to enable communication signals to pass over the bus loop 1.